In a distributed processing system, a number of processors cooperate to perform the basic functions required of the system. The processors are arranged in complexes, each of which includes one or more processors. Cooperation among these processor complexes is made possible by providing facilities to permit these processor complexes to send data messages, each consisting of one or more packets of data, to each other. A local data network is one common way of interconnecting such processor complexes in order to allow free exchange of data packets among arbitrary ones of the processor complexes in the distributed processing system. The ETHERNET.RTM. system is one common type of local data network used for permitting the processor complexes of a distributed processing system to communicate with each other.
A major objective of a distributed processing system is greater reliability of the system. These systems are frequently arranged so that no one processor or processor complex is critical to the operation of the system; functions can be flexibly assigned so that if one processor fails, other processors may take over its work. In order to have a reliable system however, the communication arrangement between processor complexes must be equally reliable. The known communication arrangements have facilities for insuring a substantial level of reliability. A coaxial cable is commonly used as the communication medium between the processor complexes. The interface circuits between a processor complex and the coaxial cable are designed so that faulty circuitry is unlikely to place a short circuit across the communication medium. It is more difficult, however, to protect against the problem of "babbling buses" in which a faulty processor complex continuously transmits data bits to the communication medium so that no other processor complex can transmit information over the medium. Further, a fault in the coaxial cable would render such a communication system totally ineffective. In such a system it is also difficult to protect against the situation in which one of the processor complexes generates an unduly large volume of data packets thereby precluding other processor complexes from normal use of the bus. Growth in a system which must maintain continuous service also presents difficulties. A problem therefore exists in the prior art in that a very high reliability local data network has not been achieved.